The invention concerns a magnet system with a magnet arrangement, a working volume and a pulse tube cooler with magnetic regenerator material.
The use of pulse tube coolers to cool elements in a magnet arrangement is disclosed e.g. in EP 0 905 434. The pulse tube cooler therein is used to cool the helium bath to temperatures below 4.2 K. The magnet coils located therein can generate a higher field than at 4.2 K, since lower temperatures permit higher current flow through the superconductor.
U.S. Pat. No. 6,029,458 discloses a magnet arrangement with a piston refrigerator which comprises a slider made of materials having a magnetic phase transition. A superconducting sleeve surrounds the refrigerator to shield the disturbing fields generated by motion of the slider, and is disposed between the cold end of the refrigerator and the volume under investigation. The superconducting sleeve thereby surrounds 90° to 270° of the cold end of the refrigerator. The magnetic disturbing field of the regenerator material induces an opposite magnetic field in the sleeve such that the magnetic flux through the sleeve is kept constant. Thereby the disturbing field is compensated for. It has, however, turned out that arrangements of this type are highly susceptible to quenches and therefore do not provide reliable and stable shielding. Fields parallel to the sleeve can moreover not be compensated for, since adequate shielding currents cannot flow due to the incomplete periphery of 90° to 270°.
EP 0 905 524 discloses a magnet system with a superconducting magnet arrangement and a pulse tube cooler. The pulse tube cooler of this magnet system is disposed in a suspension tube of the system. It can re-liquefy an evaporating cryogenic liquid.
Pulse tube coolers vibrate much less than other coolers, which is a great advantage for magnet systems which are susceptible to disturbances. To permit cooling of low-temperature superconductors, temperatures of 4.2 K and below are of interest. Pulse tube coolers with magnetic regenerator materials have a particularly high cooling efficiency in this temperature range. In particular, rare earths in the regenerator material of a pulse tube cooler provide good cooling efficiency even at temperatures down to 4.2 K. This good cooling performance is due to the large thermal capacity of the rare earths in the vicinity of their magnetic phase transition. Due to this phase transition, the rare earths are magnetized by the stray field of the magnet arrangement. The magnetic properties of the regenerator material also change with temperature and within a cooling cycle. The magnetization in the stray field of the magnet arrangement therefore changes periodically and this magnetization fluctuation causes a temporally varying electromagnetic disturbance in the working volume of the magnet arrangement. This time dependent magnetization was not disclosed in the EP 0 905 434 patent document, since this document assumes that the magnetization of the regenerator material only generates a static homogeneity disturbance in the working volume, which would not present a problem.
It is the underlying purpose of the present invention to provide a magnet system which has a pulse tube cooler with a magnetic regenerator material which nevertheless realizes a temporally constant magnetic field in the working volume with optimum stability.